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Jeep Wrangler YJ
YJ General Discussion
1990 YJ is a pain to start in the morning
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<blockquote data-quote="Gary747" data-source="post: 428321" data-attributes="member: 19207"><p>To be honest, my experience with my 1990 4.2 BBD carburetor system drove me to convert it to fuel injection. The vacuum system is extremely complex, and over the years, many of the components (check valves, vacuum motors, sensors) are no longer available new.</p><p></p><p>A couple of thoughts:</p><p></p><p>The choke circuit utilizes both vacuum and electric control. During start, with the throttle valve closed, vacuum above the throttle valve causes a richer-than-normal flow from the main-metering and idle fuel circuits, enrichening the fuel-air mixture. The choke shaft is connected to a bi-metallic coil mounted on the side of the carburetor inside a little black circular plastic cover. To gain access to that mechanism, you must drill or grind the rivets holding the cover on. This is not designed for easy access, but I did remove this cover which then allows some rotation adjustment capability. You have to come up with your own screws to put it back together. In fact, I believe you must remove the carburetor to work in this area. I had my carburetor off MANY times and replaced it once.</p><p></p><p>The choke cover has an electric heater that warms the bi-metallic coil and opens the choke. This heater is powered through an oil pressure switch... when the oil pressure is below 4 psi, no power is supplied. When oil pressure is above 4 psi, the coil is powered.</p><p></p><p>A higher RPM idle speed is built in to theoretically prevent stalling during warm up. There is a fast-idle cam controlled by the choke shaft that rotates against a fast-idle adjustment screw. The cam is stepped to provide a correct idle speed proportional to the choke valve opening. When the choke valve is fully open, the cam rotates free and the engine goes to it's normal idle speed. This cam is accessible with the air cleaner removed.</p><p></p><p>If you crank the engine with full throttle (pedal to the floor), a "tang" on the throttle lever causes the choke to open. This open position is not wide open, but a built-in open distance normal for operation. This allows starting with no choke, or clearing a flooded condition.</p><p></p><p>Now, it gets complicated. Idle speed control varies with intake manifold heater on or off, air conditioner and/or rear window defogger in use and air cleaner temp. These parameters are used by the Sole-Vac throttle positioner which is operated two ways.. an electric holding solenoid or vacuum actuator. It goes downhill from there. Just too complicated.</p><p></p><p>Also be aware of the two tapered metering pins which I recall are visible when looking down the throat of the carburetor with a flashlight. You can see them move with throttle changes. These are computer controlled so as to regulate the proper fuel/air ratio. They are driven by an externally mounted stepper motor.</p><p></p><p>Another "bad running or starting" issue I encountered was caused by the EGR system, which gets into air cleaner and coolant temps. Again, much dependence on vacuum, not to mention an EGR valve that operates in a very dirty environment.</p><p></p><p>It sounds to me that you are not getting the proper fuel/air mixture when the engine is cold and are likely struggling to keep it running utilizing the carburetor acceleration pump until it warms up. Possibly an issue with the fast idle cam I mentioned earlier. It may not be stepping as intended. Regardless, you can look forward to some invigorating trouble-shooting, possibly a carburetor replacement, and a journey through Vacuum Line Jungle.</p><p></p><p>By the way, my only regret in converting to fuel injection is that I didn't do it earlier. That conversion kit is no longer available, but I would encourage you to look at installing a 1994-ish vintage 4.0 and it's associated computer system. If not, I believe you will be forever chasing operability problems. In fact, I don't know how your Jeep got through California emissions testing until now. Somebody must have been keeping things up. I have to wonder that since you bought your Jeep recently, did the previous owner say anything about the Smog testing history?</p></blockquote><p></p>
[QUOTE="Gary747, post: 428321, member: 19207"] To be honest, my experience with my 1990 4.2 BBD carburetor system drove me to convert it to fuel injection. The vacuum system is extremely complex, and over the years, many of the components (check valves, vacuum motors, sensors) are no longer available new. A couple of thoughts: The choke circuit utilizes both vacuum and electric control. During start, with the throttle valve closed, vacuum above the throttle valve causes a richer-than-normal flow from the main-metering and idle fuel circuits, enrichening the fuel-air mixture. The choke shaft is connected to a bi-metallic coil mounted on the side of the carburetor inside a little black circular plastic cover. To gain access to that mechanism, you must drill or grind the rivets holding the cover on. This is not designed for easy access, but I did remove this cover which then allows some rotation adjustment capability. You have to come up with your own screws to put it back together. In fact, I believe you must remove the carburetor to work in this area. I had my carburetor off MANY times and replaced it once. The choke cover has an electric heater that warms the bi-metallic coil and opens the choke. This heater is powered through an oil pressure switch... when the oil pressure is below 4 psi, no power is supplied. When oil pressure is above 4 psi, the coil is powered. A higher RPM idle speed is built in to theoretically prevent stalling during warm up. There is a fast-idle cam controlled by the choke shaft that rotates against a fast-idle adjustment screw. The cam is stepped to provide a correct idle speed proportional to the choke valve opening. When the choke valve is fully open, the cam rotates free and the engine goes to it's normal idle speed. This cam is accessible with the air cleaner removed. If you crank the engine with full throttle (pedal to the floor), a "tang" on the throttle lever causes the choke to open. This open position is not wide open, but a built-in open distance normal for operation. This allows starting with no choke, or clearing a flooded condition. Now, it gets complicated. Idle speed control varies with intake manifold heater on or off, air conditioner and/or rear window defogger in use and air cleaner temp. These parameters are used by the Sole-Vac throttle positioner which is operated two ways.. an electric holding solenoid or vacuum actuator. It goes downhill from there. Just too complicated. Also be aware of the two tapered metering pins which I recall are visible when looking down the throat of the carburetor with a flashlight. You can see them move with throttle changes. These are computer controlled so as to regulate the proper fuel/air ratio. They are driven by an externally mounted stepper motor. Another "bad running or starting" issue I encountered was caused by the EGR system, which gets into air cleaner and coolant temps. Again, much dependence on vacuum, not to mention an EGR valve that operates in a very dirty environment. It sounds to me that you are not getting the proper fuel/air mixture when the engine is cold and are likely struggling to keep it running utilizing the carburetor acceleration pump until it warms up. Possibly an issue with the fast idle cam I mentioned earlier. It may not be stepping as intended. Regardless, you can look forward to some invigorating trouble-shooting, possibly a carburetor replacement, and a journey through Vacuum Line Jungle. By the way, my only regret in converting to fuel injection is that I didn't do it earlier. That conversion kit is no longer available, but I would encourage you to look at installing a 1994-ish vintage 4.0 and it's associated computer system. If not, I believe you will be forever chasing operability problems. In fact, I don't know how your Jeep got through California emissions testing until now. Somebody must have been keeping things up. I have to wonder that since you bought your Jeep recently, did the previous owner say anything about the Smog testing history? [/QUOTE]
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Jeep Wrangler YJ
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1990 YJ is a pain to start in the morning
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